Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3

Production of coated and free-standing engineering components using the HVOF (High Velocity Oxy-Fuel) process

The present study aims to establish the potential of producing various hard metal industrial thick components using the High Velocity Oxy-Fuel (HVOF) thermal spray process, rather than by sintering or casting techniques currently used. It is innovative in that generally research work earned out on this process focuses on coating technologies. In order to spray-form thick tungsten carbide cobalt (WC-Co) components, certain problems have to be overcome. More specifically these problems include minimizing residual stresses (which cause shape distortion in the components), therefore maintaining the integrity of the deposit on a microstructural scale. Residual stress arises during deposition and, in the present research, was reduced by limiting the rise and fluctuation of the deposition temperature. This was achieved by the utilization of a carbon dioxide cooling system and automated traverse movement of the spray gun, which together enabled continuous deposition at a steady temperature of 500°C, reducing the residual stress by 58% compared to manual spraying. A spraying distance of 200mm and a powder feed rate of 38gmm 1 exhibited the lowest stress within the deposit using the automated system. The minimization of the residual stress increased the maximum deposit thickness achieved from 0 6mm to 1 2mm. Higher deposit thicknesses were achieved when the deposition area was reduced. These findings were simulated by the finite element analysis technique The present research describes the successful production of 9mm thick spray-formed WC-Co components (12mm m diameter) Associative work in this thesis includes the fabricating of larger more complex shaped components, tensile specimens. The tensile specimens were used to measure the stiffness of the deposit (substrate free) and a Young’s modulus of 189GPa was measured Post-heat treatment and measurement of Vicker’s hardness and porosity were also investigated. Finally, the economic feasibility of producing WC-Co cylindrical components using spray-forming is briefly discussed

Rolling contact fatigue of thermal spray coatings

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The practical advantages of thermal spray coatings like high deposition rates, low cost and tribological properties of high wear resistance have enabled these coatings to become an integral part of aircraft and automobile industry. Recent advancements in thermal spraying techniques like high particle speed and temperature call for new applications for these coatings. This experimental study addresses the Rolling Contact Fatigue performance of thermal spray coatings deposited by a variety of techniques like High Velocity Oxy-Fuel (HVOF), Detonation Gun (D-Gun) and Plasma spraying. RCF tests were conducted using a modified four ball machine in conventional steel ball bearing and hybrid ceramic bearing configurations. Tribological conditions during the RCF tests were varied by changing the test lubricant and the lubrication mechanism, contact load and shape of the drive coated rolling element to vary the roll/slip ratio. RCF tests were analyzed on the basis of the performance, coating failures using surface and subsurface observations, and residual stress studies. Experimental and theoretical studies of the ball kinematics have also been included. These tests revealed that the performance of the coated rolling elements was dependent upon the coating and the substrate properties. The coating thickness, substrate hardness, tribological conditions during the test, coating and substrate material as well as the coating process and the substrate preparation significantly affect the coating performance and the failure modes. Three different failure modes of these coatings have been discussed along with the changes in the near surface residual stress behaviour of the coated rolling elements

Development of Graphene-Filled Fluoropolymer Composite Coatings for Condensing Heat Exchangers

Low-temperature waste heat recovery employs condensing heat exchangers to recover both sensible and latent heats. Due to the condensation of flue gases within these heat exchangers, they are subjected to severe corrosion. Perfluoroalkoxy (PFA) has been applied as a barrier layer on the surfaces of these heat exchangers for the prevention of corrosion. However, PFA has exhibited poor thermal properties and durability, which are requirements in low-temperature heat recovery applications. In this thesis, carbon-based nano-materials (8 nm and 60 nm thickness graphene particles and multi-walled carbon nanotubes, MWCNT) were incorporated into fluoropolymer (PFA) powders to generate thermally conductive and corrosion resistant composites as heat exchanger coatings. The microstructure, thermal conductivity, electrical conductivity of these composites were characterized. It was found that the thermal conductivities of the graphene-filled composites were significantly higher than that of the virgin PFA, i.e. approximately 8 times, while the composites containing MWCNT particles exhibited minimal improvement in thermal properties. The coatings containing both grades of graphene exhibited good surface finish and coating adhesion, good wear resistance and excellent corrosion resistance. The MWCNT-filled composites showed poor surface finish, resulting in poor corrosion resistance

Optimisation of the HVOF thermal spray process for coating, forming and repair of components

The High Velocity Oxy-Fuel (HVOF) Thermal Spraying technique has been widely adopted in many industries due to its flexibility, and cost effectiveness in producing superior quality of coating. The demand of high-technology industries and the availability of new advanced materials have generated major advances in this field. The HVOF thermal spray process has been utilised in many industries to apply coatings on components to protect against wear, heat and corrosion, and also to build up worn components. This spraying technology is not limited to coating substrates but also encompasses the manufacture of net shaped component from materials which are sometimes difficult to form by conventional methods. A knowledge of coating properties, testing and evaluation methods is essential in order to apply coating technology to a specific application. While spraying parameters and substrate surface preparations directly impact the coating properties, it is equally important to know the spraying technique required to deposit coating having these properties and the processing parameters which have to be applied. The thesis reports the development and optimisation of the HVOF thermal spray process for coating, forming and repair of components. A die was designed to manufacture free standing WC-Co inserts, and a similar technique was then followed to fabricate free standing annular rings and solid discs. The effects of spraying parameters on the components properties such as residual stresses and hardness were investigated and limitations identified. Experiments to assess the coatings properties involved the combinations of three spraying powders, (1) Austenitic stainless steel (2) WC-Co and (3) Tool steel match powder on stainless steel 316L andD2 tool steel substrates. Investigations were carried out on the effect of spraying distance, sprayed coating thickness and pre-spray heat treatment on coating properties including hardness, bond strength and residual stress. Results reveal that there are strong correlations between the bonding strength, coating thickness and residual stress in coatings. The tensile residual stresses coupled with increasing coating thickness cause the degradation of bond strength with increasing coating thickness. Optimisation of the repair of damaged components using the HVOF technique involved the use of similar combinations of powder and substrate materials. Tests were carried out to identify the adhesion strength of the repaired material sprayed under various conditions which were varied, including (1) repair thickness (2) prerepair and post-repair heat treatment (3) repair wall angle and (4) substrate surface preparation. In addition, the finish machining possibility of these repaired components was evaluated

Durability of multi layered plasma and HVOF coatings

The aim of this research is to investigate the effect of variable compositional coating layers on their mechanical and chemical behaviour under certain environments typically experienced in the Oil and Gas Industry. The research centers on using thermal spray coating techniques such as the HVOF and plasma spray equipment. The coating was applied onto selected carbon steel substrates (API-5L, Schedule-40, Grade-B) to simulate the material application used in oil and gas components that subject to corrosion and corrosion-erosion phenomena. The coating was deposited using three layers of coatings with various combinations of sprayed powders to create graded multi-layered coating. WC-12Co powders with two different powder structures (micro-sized powders and nano-sized agglomerated into micro-sized powders – nano-structured) were mixed at different ratios with AMDRY (9954 and 995M) powders to enhance the mechanical behaviour, wear, and corrosion protection (Hardness, corrosion, erosion-corrosion) of the coatings. The surface morphology and the elemental composition of the coatings were examined using SEM and EDS techniques. The results indicated that coating deposited by the HVOF (60% Micro WC-12Co and 40% Micro AMDRY 9954) was the most effective coating system in preventing corrosion. If the erosion is the dominant concern, then replacing the Microsized WC-12Co powder with the Nanosized WC-12Co powder is effective for protecting materials from wear caused by erosion-corrosion. The addition of the AMDRY powder to the original WC-Co powders helped to lower the overall coating porosity, enhanced the corrosion protection, and improved the overall erosion–corrosion performance, which addresses issues currently faced in the Oil/Gas industry. A Design of Experiment (DOE) technique may be used to optimize the powders blending ratios to give better results in future work

Development of Polymer Composite Coatings for Condensing Heat Exchangers

Polymer coatings exhibit superior corrosion resistance, making them a good solution to protect heat exchange components from chemical attack in low temperature heat recovery. Nonetheless, major shortcomings to using polymer coatings include their low thermal conductivity, low strength, and susceptibility to wear. Studies conducted collaboratively by the University of Windsor and CanmetMATERIALS have investigated the suitability of perfluoroalkoxy (PFA) composite coatings with conductive filler materials, such as graphene, for polymer composite coatings. The following research investigates the impact of two different filler incorporation techniques, ball milling and magnetic functionalization, to optimize the microstructural, thermal, and tribological properties of the polymer composite coating. Through microscopy, the investigation revealed that the ball milled samples display excellent filler particle distribution, and a general lateral alignment of graphite filler particles. The composites displayed a decrease in the thermal conductivity after ball milling, resulted from the lateral alignment of filler particles and measurement of the thermal properties in the out-of-plane direction. Furthermore, Raman analysis indicated that the ball milling process did not produce monolayer graphene. The magnetically functionalized multi-layer graphene (MF-MLG) particles were responsive to an external magnet however, microscopy showed that the MF-MLG were not aligned within the polymer matrix. A combination of abrasive and adhesive wear was observed through pin-on-disk wear testing; higher weight fractions of filler resulted in lower wear resistance. All composites displayed very low coefficient of friction values throughout testing

Remanufacturing and Advanced Machining Processes for New Materials and Components

"Remanufacturing and Advanced Machining Processes for Materials and Components presents current and emerging techniques for machining of new materials and restoration of components, as well as surface engineering methods aimed at prolonging the life of industrial systems. It examines contemporary machining processes for new materials, methods of protection and restoration of components, and smart machining processes. • Details a variety of advanced machining processes, new materials joining techniques, and methods to increase machining accuracy • Presents innovative methods for protection and restoration of components primarily from the perspective of remanufacturing and protective surface engineering • Discusses smart machining processes, including computer-integrated manufacturing and rapid prototyping, and smart materials • Provides a comprehensive summary of state-of-the-art in every section and a description of manufacturing methods • Describes the applications in recovery and enhancing purposes and identifies contemporary trends in industrial practice, emphasizing resource savings and performance prolongation for components and engineering systems The book is aimed at a range of readers, including graduate-level students, researchers, and engineers in mechanical, materials, and manufacturing engineering, especially those focused on resource savings, renovation, and failure prevention of components in engineering systems.

Investigation into laser re-melting of inconel 625 HVOF coating blended with WC

High velocity oxy-fuel (HVOF) spraying of Diamalloy 1005 powders mixed with WC particles onto steel (304) is considered and laser re-melting of the resulting coatings is examined. Laser re-melting process is modeled to determine the melt layer thickness while temperature increase is formulated using the Fourier heating law. The morphological and metallurgical analyses prior and post laser re-melting process are carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) technique is used to determine the residual stress developed in the coating while the analytical formulation is adopted to predict the residual stress levels at the coating base material interface. The indentation tests are carried out to determine the Young’s modulus and fracture toughness of the coating prior to laser re-melting. Corrosion resistance of coating is measured using potentiodynamic polarization technique prior and post laser treatment process. The predictions of the melt layer thickness are in good agreement with experimental results. The presence of WC particles modifies temperature rise and its gradient in the coating while affecting the Young’s modulus, residual stress levels, and fracture toughness of the coating. The differences in the thermal properties of Inconel 625 powders and WC particles result in formation of small size cellular structure through polyphase solidification. WC dissolution in the central region of the large polycrystalline cells is observed due to the loss of carbon through carbonic gas formation. The results of corrosion tests prevail that significant improvement of corrosion resistance can be achieved after laser treatment process